In nonequivalent group design, the researcher chooses existing groups that appear similar, but where only one of the groups experiences the treatment. Here we explain three of the most common types: nonequivalent groups design, regression discontinuity, and natural experiments. Many types of quasi-experimental designs exist. You can use these pre-existing groups to study the symptom progression of the patients treated with the new therapy versus those receiving the standard course of treatment.Īlthough the groups were not randomly assigned, if you properly account for any systematic differences between them, you can be reasonably confident any differences must arise from the treatment and not other confounding variables. Example: Quasi-experimental designYou discover that a few of the psychotherapists in the clinic have decided to try out the new therapy, while others who treat similar patients have chosen to stick with the normal protocol. Instead, you can use a quasi-experimental design. In this case, you cannot run a true experiment. However, for ethical reasons, the directors of the mental health clinic may not give you permission to randomly assign their patients to treatments. The other half-the control group-receives the standard course of treatment for depression.Įvery few months, patients fill out a sheet describing their symptoms to see if the new treatment produces significantly better (or worse) effects than the standard one. Example: True experimental designTo run a true experiment, you randomly assign half the patients in a mental health clinic to receive the new treatment. Requires the use of control and treatment groups.Ĭontrol groups are not required (although they are commonly used).Įxample of a true experiment vs a quasi-experiment Let’s say you are interested in the impact of a new psychological therapy on patients with depression. The researcher often does not have control over the treatment, but instead studies pre-existing groups that received different treatments after the fact. The researcher usually designs the treatment. Some other, non-random method is used to assign subjects to groups. The researcher randomly assigns subjects to control and treatment groups. There are several common differences between true and quasi-experimental designs. Frequently asked questions about quasi-experimental designsĭifferences between quasi-experiments and true experiments.Differences between quasi-experiments and true experiments.
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